Motor vehicles, as presently manufactured, are equipped with systems for defogging and deicing windshields associated therewith. Generally, the defogging and deicing systems depend upon heat generated in the internal combustion engine of the vehicle and subsequently transferred to the engine's cooling system. This heat generated by operation of the internal combustion engine is blown as warm air across the interior of the windshield to accomplish a defogging and/or deicing thereof.
In such a case, that is, when heat generated in the internal combustion engine is used for defogging and/or deicing, it is apparent that such operation will take a period of time before it can be completed because of the inherent delay built into the process. The delay occurs because there is a period of time between the starting of an internal combustion engine and the time that sufficient heat is being generated in its cooling system to provide the heat required for defogging and/or deicing of the vehicle's windshield. Depending upon the exact temperature conditions, the amount of buildup of frozen material on the windshield, and the time the vehicle has been sitting idle without its engine running, the period before sufficient heat is available from the engine's cooling system can be up to 10 minutes or more.
In view of the fact that there can be a rather lengthy delay before the present-day vehicle's heating and defrosting system can clear a windshield from material which is frozen thereon, more effective systems have been sought. Recently, automotive designers have designed systems which generate heat from electrical energy to accomplish a relatively rapid defrost and/or deicing of a vehicle's windshield. An electrically heated defrosting and deicing system of this type is now found on vehicles produced by Ford Motor Company. Such an electrically heated defrosting and deicing system is independent of the normal heating and defrosting system found in the motor vehicle.
Heat which is used for an electrically heated defrosting and deicing system for a windshield is generated by flowing an electric current through a conductive coating which has been applied to a surface of the windshield. U.S. Pat. No. 4,543,466 which issued on Sept. 24, 1985 for "Bus Bar Arrangement for Uniformly Heating a Trapezoidally-Shaped Electrically Heated Windshield", discloses a construction for an electrically heated windshield. This patent is assigned to Ford Motor Company. The patent teaches that the windshield's electrically conductive coating may be applied, for example, by a magnetron sputtering operation in which layers of metallic-containing materials are applied to a selected surface of the windshield. Suitable electrical connection to the coating materials provides the electrical path through which electrical current may be passed through the conductive coating.
As taught in the aforementioned U.S. Pat. No. 4,543,466, the materials used as target materials in the magnetron sputtering device for generating the electrically conductive coating are zinc and silver. The resulting coating applied to a selected surface of the windshield is a multilayer coating consisting of a layer of silver between layers of zinc oxide. Normally, this conductive coating is applied to the inside of the surface of the glass sheet which defines the outside of a laminated glass windshield. In particular, the surface selected for application of the conductive coating is the surface of the outer glass sheet which is bonded to a laminating interlayer to form the laminated windshield.
The application of this conductive coating, however, changes the transmittance value of the overall windshield. In other words, as more and more coating material is applied to the windshield to form the conductive coating, the windshield will transmit therethrough less and less light in the visible wavelength range. This reduction in transmittance also occurs with the placement of other types of conductive coatings for forming electrically heated windows.
In accordance with United States laws, the transmittance of a window used as a windshield for a motor vehicle must be at least 70% in the visible spectrum. Thus, the thickness of the conductive coating which insures the transmittance of the windshield is above that required by law, in turn dictates the amount of current which may flow therethrough to generate heat for the windshield. For example, if increasing the thickness of the conductive coating rapidly decreases the transmission of light in the visible spectrum therethrough, then the conductive coating must remain relatively thin so that the overall transmittance of the windshield in the visible spectrum remains at least 70% or more. However, if the coating is relatively thin, then the amount of current which can flow therethrough per unit of time is also limited and as a result the heating of the windshield for defogging and/or deicing purposes will take a longer period of time.
In accordance with our inventive method and product produced thereby, we improve the transmittance of a window in the visible transmittance range even though it has an electrically conductive coating thereon. This results in either one of two benefits. As a first benefit, we can achieve a higher transmittance through a window having a given thickness of electrically conductive coating thereon. As a second benefit, we may keep the same visible transmittance in a window and increase the thickness of the conductive coating thereon whereby a more rapid deicing and/or defogging of the electrically conductive window can be achieved.
If one desires to follow the first alternative, then one may achieve transmittance levels through the window, such as a laminated windshield, up to 75% or more of the light transmitted in the visible spectrum. The 75% transmittance level is the European standard. By that we mean that windshields produced for European markets must transmit at least 75% or more of the light incident thereon in the visible spectrum in order to be acceptable. Windshields which have only a 70% transmittance in the visible spectrum, although acceptable for the U.S. market, are not acceptable for European markets. However, by following the method of our invention, the same thickness coating of electrically conductive material that will produce 70% transmittance in an electrically heated window such as a windshield for the U.S. market will now produce 75% transmittance in the visible spectrum and will be suitable for sale in European markets.
In the alternative, here in the U.S. where 70% transmittance of light in the visible spectrum is acceptable for a laminated windshield, the thickness of the conductive coating on such a windshield may be increased when the electrically heated windshield is made in accordance with our invention. Even though a thicker coating of conductive material is now used in the electrically heated windshield, the overall transmittance stays at 70% or more if the method of our invention is followed. In such a case, more current may be flowed through the thicker conductive coating associated with the electrically heated windshield, and the windshield will be more rapidly defrosted and/or deiced.
In summary, our method is one which improves the transmittance of an electrically heated window, such as a windshield, having a given thickness of conductive coating thereon, or, if one desires the same transmittance in an electrically heated window, such as a windshield, a thicker coating of conductive material may be placed thereon. The product produced by our improved method is also of value since its transmittance of light in the visible wavelength is increased over that which it would have if our method had not been applied thereto.
No search was conducted in the U.S. Patent and Trademark Office or in any other search facility on the subject matters disclosed herein as our inventions.